Above a waterfall, a humanoid alien consumes a dark liquid, and a hovering spacecraft departs. After drinking the liquid the alien starts to disintegrate. The bodily remains fall into the waterfall below and the alien’s DNA triggers a biogenetic chain reaction.

2 Answers
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As we can see after he drinks the liquid, his DNA seems to desintegrate into the water. But from the particules of his DNA, life is created. It is showed by the particules turning step by step into cellular life.

So my hypothesis is that his suicide was necessary to give birth to a new species: Us.

A friend of mine told me that he might have missed his flight (we can see a ship leaving), but I don't think that it is a good reason to get suicidal.

Okay, so is he on Earth when he does this? Is it thousands of years ago?
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Evik JamesJun 11 '12 at 16:17

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@EvikJames what I took from this opening scene was that groups of Engineers go to any planet that can support life (as they see it) and perform this ritual. That being said, this could be Earth, or it could be another planet, millions of years ago or quite recently. Remember that the film is named after Prometheus, the Titan from Greek mythology who stole fire from the Gods and gave it to the humans. This could be allegorical for stealing life (from their creators/god(s)) and giving it to lower forms of life.
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Jamie TaylorJun 12 '12 at 8:06

Scott has stated in interviews that the planet at the beginning "could be any planet," meaning that the Engineers have probably terraformed many planets and seeded them with life via self-sacrifice.
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David HarknessJun 22 '12 at 3:36

It can be assumed that the Humanoid or "Space Jockey" at the beginning
of the movie was standing on Earth, and the liquid that he drank was
probably the same matter that David discovers (when he says "big
things have small beginnings"). Supposedly this liquid dematerialises
the Space Jockey's DNA which eventually combined with Earth's water
gives rise to all living beings on earth, including humans. Why the
Space Jockeys couldn't have extracted DNA from stem cells or even just
manufactured it, remains a mystery. One theory is that that along with
the mythology behind the movie title, the space jockey standing on
Earth stole the technology from the biological weapons seen on LV-223,
to create new life instead of destruction. This lone dissident having
no means to add the space jockey/human genome to the liquid compound
had to make do with what he had; his own body. Concept art of the film
showed an extended opening scene involving two Space Jockeys. An elder
SJ gives a younger looking SJ the cup of black liquid in a ritualistic
manner. So the opening scene is part of a ritual to begin the creation
of organic life on any given planet. The scene is rumored to have been
shot so it may end up on the home release as behind the scenes or
deleted footage.